电容器的介电极化

Polar dielectrics have permanent charge separation in their molecules, creating a permanent dipole moment. Nonpolar dielectrics lack this charge separation but become polarized when exposed to an external electric field. In both cases, dipoles are randomly oriented without an applied field, but polar molecules align more readily with the field.

When an external electric field is applied to a nonpolar dielectric, it separates the positive and negative charges within the molecules, inducing polarization. This induced polarization aligns the charges along the field direction. The nonpolar molecules effectively become polar in the presence of the external field.

The induced surface charges in the dielectric create an induced electric field that opposes the external field. The net electric field is the vector sum of the external and induced fields, which is weaker than the original external field. This relationship is quantified using the dielectric constant.

Induced surface charges on the dielectric faces reduce the voltage across the capacitor plates and increase the overall capacitance. These charges create an induced electric field that opposes the external field, effectively weakening the net field between the plates and enhancing the capacitor's ability to store charge.

Dielectric breakdown occurs when the external electric field magnitude exceeds a critical limit, ionizing molecules in the dielectric material. This ionization produces free electrons, allowing current to flow between the capacitor plates through the dielectric. The material then behaves like a conductor, compromising the capacitor's function.

Polar dielectrics have randomly oriented dipoles that align with the applied uniform electric field. This alignment induces equal and opposite surface charges on the dielectric faces. The organized dipole orientation reduces the net electric field between the capacitor plates and increases capacitance.

The net electric field in a dielectric is the vector sum of the external field and the induced field from surface charges. This relationship is expressed mathematically using susceptibility permittivity and dielectric constant, which quantifies how much the material reduces the electric field compared to vacuum.